5.2. Configuring gdm

gdm as a complete rewrite of xdm uses its own configuration file: /etc/X11/gdm/gdm.conf

you should locate the definitions of the local X servers and add additional X servers for the number of cards you have.

modifications:

5.2.1. changes part 1/2

.......
[servers]
# These are the standard servers.  You can add as many you want here
# and they will always be started.  Each line must start with a unique
# number and that will be the display number of that server.  Usually just
# the 0 server is used.
0=Standard
1=Second
.......
here you'll have only 0=Standard, for each additional X server you should add a definition like here for 1=Second

5.2.2. changes part 2/2

..............................
# Definition of the standard X server.
[server-Standard]
name=Standard server
command=/usr/X11R6/bin/X0 :0 -deferglyphs 16 -ac vt7
flexible=true

[server-Second]
name=Second server
command=/usr/X11R6/bin/X1 :1 -deferglyphs 16 -xf86config /etc/X11/XF86Config-4.[your card] vt9
flexible=true
..............................   
and here the exact definition of the command line for starting the X server, very similar to the definitions in /etc/X11/xdm/Xservers (the definition of [server-Second] is the additional one)

In case you use single XFree configuration file
..............................
# Definition of the standard X server.
[server-Standard]
name=Standard server
command=/usr/X11R6/bin/X0 :0 -layout first-Xserver -deferglyphs 16 -ac vt7
flexible=true

[server-Second]
name=Second server
command=/usr/X11R6/bin/X1 :1 -layout second-Xserver -deferglyphs 16 vt9
flexible=true
..............................